6o4 



NATURE 



[October 21, 1897 



THE AGENCY OF MAN IN THE DISTRIBU- 

 TION OF SPECIES} 



A MONO the many influences which, during the last century 

 "^ or two, have been affecting that unstable condition of life 

 which is expressed in the words " the geographical distribution 

 of animals and plants," none has approached in potency the 

 agency of man, exerted both purposely and unwittingly or acci- 

 dentally. 



Natural spread was for centuries the rule. Species dispersed 

 under natural conditions along the line of least resistance. 

 Winged animals and seed were spread by flight and by the 

 agency of winds, and at their stopping-places thrived or did not 

 thrive, according as conditions were suitable or not suitable. 

 Aquatic animals and plants and small land animals and plants 

 were distributed by the action of rivers and streams and by the 

 ocean itself. Wonderful migrations have occurred, commonly 

 with birds, more rarely with other animals ; ice-floes and drift- 

 wood have carried animals and plants far from their original 

 habitats, and even volcanic action has taken part in the dis- 

 persal of species. Smaller animals, especially molluscs and 

 insects, and the seeds of plants have been carried many hundreds 

 of miles by birds, and lesser distances by mammals. 



With the improvement of commercial intercourse between 

 nations by land and by sea another factor became more and 

 more prominent, until- in the present period of the world's 

 history the agency of man in the spread of species, taking all 

 plant and animal life into consideration, has become the pre- 

 dominating one. Potentially cosmopolitan forms, possibly even 

 insular indigenes, have by this important agency become dis- 

 persed over nearly all of the civilised parts of the globe, while 

 thousands of other species have been carried thousands of miles 

 from their native homes, and have established themselves and 

 flourished, often with a new vigour, in a new soil and with a 

 novel environment. 



It is obvious that this agency is readily separable into two 

 divisions — intentional and accidental. 



Intentional Importations. 

 Since early times strange plants and animals have been 

 carried home by travellers. Conquering armies have brought 

 back with the spoils of conquest new and interesting 

 creatures and useful and strange plants. With the discovery 

 of America and with the era of circumnavigation of the globe 

 such introductions into Europe of curious and useful species, 

 plants in particular, increased many-fold, while with the 

 colonisation of America and other new regions by Europeans 

 there were many intentional return introductions of Old World 

 species conducive to the welfare or pleasure of the colonists. 

 Activity in this direction has been increasing and increasing. 

 Public botanical gardens and many wealthy individuals in all 

 quarters of the globe have hardly left a stone unturned in their 

 efforts to introduce and acclimatise new plants, particularly 

 those of economic importance and nesthetic quality, not failing 

 occasionally, it must parenthetically be said, to establish some 

 noxious weed, or some especially injurious insect ; while it is 

 safe to say that probably the majority of the desirable 

 plants of Europe which will grow in the United States have 

 already been introduced, and that there has been an almost 

 corresponding degree of activity in the introduction of desirable 

 plants from the United States into Europe. In all this host of 

 valuable introductions there have been comparatively few which 

 have turned out badly, aside from failures of establishment. 

 The wild garlic {Allhiin vineale), that ubiquitous plant which 

 gives its taste to milk, butter, and even to beef during 

 the spring and summer months in many States, is said to 

 have been intentionally introduced by the early residents of 

 Germantown, Pennsylvania. The water hyacinth {Ptaroptis 

 crassipes), originally grown for ornament in a pond near Palatka, 

 Florida, escaped into the Saint John's River about 1890, and has 

 multiplied to such an extent as to seriously retard navigation 

 and to necessitate Government investigation. The distribution 

 of the orange hawk-weed {Hieraciuiu aurantiacum), a dan- 

 gerous species which has ruined hundreds of acres of pasture 

 land in New York of recent years, was originally aided by a 

 florist as a hardy ornamental plant. The European woad-waxen 

 {Genista tinctorium) was early introduced at Salem, Mass., in 



•1 Abr idged from an address by Dr. L. O. Ho A'ard, printed in Science of 

 Septem ber 10. 



NO. 1460, VOL. 56] 



fact about thirty years after the settlement of the colony. It 

 has apparently not been used as a dye plant, but for garden and 

 ornamental purposes only. During the last few years it has 

 become a noxious weed throughout Essex and the adjoining 

 counties. Standing recently on a rock at Swampscott, the 

 writer was able to see that the country for miles around was 

 coloured a bright yellow with enormous masses of this plant. 

 Similar instances are fortunately rare, and the majority of our 

 noxious weeds have been accidental introductions. 



Intentional introductions of animals, however, have by no means 

 resulted as advantageously as intentional introductions of plants, 

 with the exception of the truly domesticated species, such as 

 the horse, ass, cow, sheep, pig, dog, cat, poultry, honey-bee, 

 and silk-worm of commerce. Even with such species, the 

 grazing ranges of Australia have been overrun by wild horses 

 to such an extent that paid hunters shoot them at a small sun> 

 per head, and the European rabbit has become a much wor.se 

 plague on the same island continent. 



Intentional introductions of wild species, however, have 

 almost without exception resulted disastrously 



At various intervals between 1850 and 1867 a few pairs of 

 English sparrows were introduced into the north-eastern States 

 to destroy canker-worms, and to-day this species is an ubiquitous 

 and unmitigated pest throughout all the austral and transition 

 regions of North America, finding its limit only at the borders 

 of the boreal zone, while the place of the injurious insect it was 

 imported to destroy has been taken by another and worse insect 

 pest which it will not touch. 



In 1872 Mr. W. Bancroft Espeut imported four pairs of the 

 Indian mongoose from Calcutta into Jamaica for the purpose ol 

 destroying the " cane-piece rat." Ten years later it wa-; 

 estimated that the saving to the colony through the work of 

 this animal amounted to ioo,ooo?. annually. Then came a 

 sudden change in the aspect of affairs. It was found that the 

 mongoose destroyed all ground-nesting birds, and that the poultry 

 as well as the insectivorous reptiles and batrachians of the island 

 were being exterminated by it. Injurious insects increased in 

 consequence a thousand-fold ; the temporary benefits of the 

 introduction were speedily wiped away, and the mongoose 

 became a pest. Domestic animals, including young pigs, kids, 

 lambs, newly-dropped calves, puppies and kittens, were 

 destroyed by it, while it also ate ripe bananas, pine-apples, 

 young corn, avocado pears, sweet potatoes, cocoas, yams, peas, 

 sugar-cane, meat, and salt provisions and fish. Now, we are 

 told, nature has made another effort to restore the balance. 

 With the increase of insects, due to the destruction by the mon- 

 gooses of their destroyers, has come an increase of ticks, which 

 are destroying the mongoose, and all Jamaicans rejoice. 



The flying-foxes of Australia {Pteropus sp ) are animals 

 which are very destructive to fruit in their native home. 

 Frequently some well-meaning but misguided person will 

 arrive on a steamer at San Francisco with one or more of those 

 creatures as pets. While it is not probable that any of the 

 flying-foxes will thrive in northern California or, in fact, in austral 

 regions, the experience is too dangerous a one to try, and the 

 quarantine officer of the California State Board of Horticulture 

 has always destroyed such assisted immigrants without mercy. 



Less than thirty years ago (in 1868 or 1869) Prof. Trouvelot 

 imported the eggs of the gypsy moth {Porthelria dispar) into 

 Massachusetts. The insect escaped from confinement, increased 

 in numbers, slowly at first, more rapidly afterwards, until in 



1889 it attracted more than local attention, with the result that in 



1890 the State began remedial work. This work has steadily 

 progressed since that time, and the State has already expended 

 something over a half-million of dollars in the effort to exter- 

 minate the insect, and it is estimated that one million five hundred 

 and seventy-five thousand dollars more must be used before 

 extermination can be effected. 



Contrast with this a single intentional importation which has 

 had beneficial results. The Australian ladybird ( Vedalia 

 cardinalis) was introduced into California in 1889 with tlie 

 result of saving the whole citrus-growing industry of the State 

 from approaching extinction through the ravages of the 

 cottony-cushion scale [Icerya purchasi). Later importations of 

 the same insect into South Africa and Egypt also resulted 

 beneficially. 



We have thus had sufficient experience with intentional im- 

 portations to enable us to conclude that while they may often he 

 beneficial in a high degree, they form a very dangerous class of 

 experiments, and should never be undertaken .without the fullest 



